The present invention relates to a semiconductor laser device usable as, for example, a light source for optical communications, optical measuring apparatuses and the like.
A distributed feedback laser diode (DFBLD) or a distributed Bragg reflector laser diode (DBRLD) which is operable in a single mode is a promising light source for high-speed and long-distance optical fiber communications Due to its good single mode purity of lasing wavelength, this laser is also suitable for use in optical measuring apparatus in which a coherent optical arrangement is built in. Indeed, in an experimental optical fiber communication system having a data rate as high as 4 Gb/s and a transmission distance of longer than 100 kg, a DFBLD using an InGaAsP/InP material has been used as a light source and has proven to be effective. Further, it has been found that a device with good characteristics exhibits in a single mode a high output CW operation (above 100 mW) and high temperature CW operation (as high as 140.degree. C.), which are comparable with the characteristics of a conventional Fabry-Perot type semiconductor laser. However, a DFBLD, unlike a Fabry-Perot type laser, cannot readily be provided with a structure which allows it to lase in single mode. Specifically, Fabry-Perot type laser devices which substantially satisfy necessary conditions can be produced with stability only if the transverse mode is controlled. With the DFBLD, however, the lasing spectrum varies in a complicated manner between the single mode and plural modes depending upon various factors such as the particular diffraction grating phase at which the diffraction grating is terminated at the emitting end of the DFBLD, making it difficult to produce devices that are stably operable in a single mode at a high yield.